I love the E screen in my F3. I also have one in my FM2n. Best all around screen for me anyway. The split image rangefinder screen is nice with wide aperture lenses, but they have an annoying tendency to partially black out with slower lenses.
Is your 300 f/4.5 the internal focusing model? If so, it's a pretty good lens. Not the best 300 mm. I've ever seen, but I have no complaints at all about it. The fact that it is relatively small and light and features internal focusing makes it one of the easier handling manual focus lenses in this focal length.